<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>flex</title>
	<style>
		.box {

			background-color: skyblue;
		}

		.box > img {
			position: relative;
			z-index: -1;
		}
	</style>
</head>
<body>
<!--
	transform的时候，就变成了层叠上下文元素。
-->
<button onclick="setStyle()">启动</button>
<div class="box">
	<img src="../../image/星空.jpg">
</div>
<script>
	function setStyle() {
		document.getElementsByClassName('box')[0].style.filter = "blur(5px)"
	}
</script>
</body>
</html>